AbstractRats were submitted to hypoxia corresponding to 6,000 m altitude. The effect on hemoglobin concentration, hematocrite, plasma protein concentration, total hemoglobin and the specific activity of the erythrocytes, tagged in vivo with Ci14‐2‐glycine has been studied during the first two days. The hemoglobin and plasma protein concentrations increased significantly. They were significantly correlated to each other. There is no indication that the spleen should act as a blood reservoir during these circumstances. The specific activity of hemin was unchanged, which indicates that no essential hemolysis or increase of the circulating hemoglobin occurred. However, the total hemoglobin increased significantly. Thus the amount of hemoglobin in the erythropoietically active tissues must have increa